I checked the hen and the liver, heart and stomach seemed normal. Fat didn't look un normal and the ovaries looked fine. I think it was her that laid that egg, there wasn't any inside that looked like she was going to lay tomorrow. Really not sure what happened
I had a chicken suddenly die last year. She was fine in the morning, even laid an egg. That afternoon I found her dead. It was hot but it wasn't the hottest day of the year. So I have no idea. Sudden unexpected death usually is caused by a problem with the heart or the brain. Heart attack or stroke or blood clot of some sort. The only way to really know is to have a necropsy done. Those can be expensive though.
I've heard people say that they've had chickens literally drop dead in front of them with no apparent cause. Maybe it is a heart attack who knows. It's very sad either way. I'm sorry about your chickens.